Confessions of a Cat Breeder

Hello everyone,
I have a litter of 5 kittens 9 days old..up until yesterday all seemed well with all of them until last night when I noticed the runt was not anywhere near the mother nor the others..breathing seemed loud. I took him into the vet today and she said he has a heart murmur and he may well survive but at this point in time nature will take it's course. She advised to make sure he eats and whenever I clear the path while the others sleep he does not eat and keeps
going back to the outside area of the box. I understand nature is nature but does anyone have any suggestions beyond watching and waiting. I feel
so helpless. Thank you for any advice. Donna